the effective strong ion difference (SIDeff)
this is the electric charge attributable to the measured weak ions, measured in mEq/l.
a "weak ion" is one that under pH conditions compatible with life is appreciably less than 100% ionised -
these are mainly albumin and phosphate. albumin has more than 100 moieties capable of dissociating, the histidin residues provide
the bulk of those being incompletely ionised, thus changing their degree of dissociation with pH - "buffering" is the more
conventional term for this. the same is true for the transition of
H2PO3-
to HPO32-.
you can calculate these dissociations playing with different pH values here.
the formulae used in this programme are based on the formulae Figge and Fencl presented in their article in AmJRCCM 2000, 162, 2246.
all of the possible approaches, inspite of their discrepant complexities, correlate very well with each other, as demonstrated by
Chris M. Anstey, Queensland, Australia (J Appl Physiol 98: 2119-2125, 2005).
the Figge- Fencl article is strongly recommended reading - it served as an introduction to the topic for me, RG.
the other two articles are quite complex, but essential to the framework validating and informing our programme.
there are several formulae used for calculating the degree of ionisation of albumin and phosphate.
the simplest one is the classical one Stewart himself used: lump all the weak acids together as HA,
take a mass equation for the reaction HA <=> H+ + A- and
determine a single dissociation constant Ka.
using this original Stewart formula
[H+]*[A-]=Ka*[HA] for human serum, a good aproximation has been
found by a veterinarian, Henry Stämpfli:
using only albumin (in g/l, with 0.378mmol/g) or total protein (in g/l, with 0.224mmol/g)
as a measured weak acid: Ka = 8*10-8Eq/l
when calculating the weak acid ionisation our scripts are mainly based on 2 formulae taken from the work of
Figge and Fencl(2), with a small correction in the albumin formula recommended by James Figge, via e-mail, 20090217,
taking advantage of the fact that the titration curve for albumin and phosphate in isolated
plasma is almost linear in the physiologically interesting pH range, and abstracting from the fact that the molecules of both
have multiple moieties capable of dissociation:
albumine charge = [albumin]*(pH*0.1204 - 0.625) (albumin in g/l)
phosphate charge = [PO4]*(pH*0.309 - 0.469) (phosphate in mmol/l)
the most advanced model of weak acid dissociation is the "Figge Fencl model", calculating the degree of ionisation separately
for all the potentially dissociating moieties - 3 for phosphate, 212 for albumin. the data for these
calculations have been taken from a publication by Chris M. Anstey. (3)
if you use the "advanced search" option on our database, you can choose to run our analysis scripts using this formula, the difference
with the results obtained with linear formulae is small, though.
James Figge maintains a website of his own with a lot more detailed information:
acid-base.
the site offers colourful representations of the albumin molecule's x-ray structure:
albumin conformation and structure.
(1) Staempfli and Constable -
Experimental Determination of Net Protein Charge and Atot and Ka of Nonvolatile Buffers in human plasma,
J Appl Physiol 95:620- 630, 2003
(2) Fencl, Jabor, Kazda, Figge -
Diagnosis of Metabolic Acid-Base Disturbances in Critically Ill Patients,
Am J Respir Crit Care Med, 2000, Vol 162, 2246- 2251
(3) Comparison of three strong ion models used for quantifying the acid-base status of human plasma with special emphasis on the plasma weak acids
Chris M. Anstey
Department of Intensive Care, Nambour Hospital, Nambour, Queensland, Australia
J Appl Physiol 98: 2119-2125, 2005
